A conventional over-pressure protection system comprises a shut-off valve that closes if the pressure in the low-pressure handling system exceeds a predetermined value, and a mechanical relief valve that opens if the pressure in the low-pressure handling system exceeds a predetermined value to protect against failures of the shut-off valve. A disadvantage of the conventional over-pressure protection system is that the high-pressure source remains connected to the low-pressure handling system in case the shut-off valve fails and the mechanical relief valve opens. Moreover, Applicant had studied the reliability of such a relief valve, and found that the number of failures per unit of time was considerable, more than 5 failures in 1000 years of service life.